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HFS-5 HFS-6 UHFS-09
HFS Heat Flux Sensor Instruction Manual
Introduction
Heat flux sensors can be used to measure and quantify the thermal energy per unit area
moving through a surface of a system and can measure heat transfer in HVAC systems,
insulation analysis, refrigeration systems as well as countless other thermal applications.
Heat Flux Sensor Description
The HFS-5, HFS-6, & UHFS-09 heat flux sensors utilize a differential-temperature
thermopile design to measure the movement of thermal energy per unit area, or heat flux,
through the sensor surface. Each HFS sensor includes an integrated type-T thermocouple
that can be used for sensor temperature measurements. The sensitivity of each sensor
is provided with each unit recorded on their respective calibration certificate. Sensor
calibration procedures adhere to ASTM standard C1130 and are described later in this
manual.
Instruction Manual Contents
1
How to Use a Heat Flux Sensor
Brief Overview of Using the Heat Flux Sensor
3
Sensor Signal Measurements
3
Measuring Heat Flux Voltage
3
Measuring Thermocouple Voltage
4
Checking the Functionality of HFS Sensor &
Troubleshooting
4
Sensor Mounting/Installation
6
Removing Sensor from Measurement Surfaces
6
2
Converting Measurements to Heat Flux and Temperature
Type T thermocouple temperature measurement
6
Temperature Dependence of HFS Heat Flux Sensor
6
Calculating Heat Flux
7
Heat Flux Sensor Sensitivity Determination
8
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3
Directive Compliances
RoHS Compliance
9
REACH Compliance
10
CE Compliance
11
List of Symbols Used in this Manual
Term Represented by
Symbol
Symbol
English Units
Metric / SI
Units
Heat Flux
q”
BTU/(ft2-hr)
W/m
2
Electrical Resistance
Ω
Ohms or kOhms
Voltage Output
ΔV
V or mV or µV
Heat Flux Sensor Sensitivity
S
µV / BTU/(ft2-hr)
µV/(W/m2)
Temperature
T
°F
°C
Temperature Difference
ΔT
°F
°C
Heat Flux Sensor Sensitivity
at a Temperature
S@ T°C
µV / BTU/(ft2-hr)
µV/(W/m2)
Sensitivity Multiplication
Factor
M.F.
No Units
Temperature Gradient
dT/dx
°F/ft
°C/m
Thickness of Material
δ
ft
m
Thermal Conductivity
λ or k
BTU/(ft2-hr)/°F
W/(m-K)
Thermal Resistance
R”
°F/BTU/(ft2-hr)
(m2-K)/W
Unit Conversion Factors
Term
Conversion Method
Heat Flux
1 W/m2 = 0.317 BTU/ft2-hr
Sensor Sensitivity
1 µV/(W/m2) = 3.155 µV/(BTU/(ft2-hr)
1 µV/(W/m2) = 10 mV/(W/cm2)
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Section 1: How to Use a Heat Flux Sensor
Below are details on how to use a heat flux sensor to take thermal measurements. These
directions are for general use and can be somewhat modified depending on the test
conditions in order to collect the most accurate measurements for your application.
Brief Overview of Using the Heat Flux Sensor
1. Ensure the sensor is operating properly by conducting simple tests of
functionality.
2. Mount the sensor to the measurement surface.
3. Connect the heat flux sensor leads and the integrated thermocouple leads to a
precision voltmeter or precision data acquisition device.
4. Collect measurements by reading the analog DC voltage signals from the wire
leads.
5. Adjust the sensitivity of the heat flux sensor according the sensor’s temperature
and the temperature dependence function (not necessary for UHFS-09 sensors).
The sensitivity of the sensor is provided with each unit on their respective
calibration certificate in addition to the temperature dependence function.
6. Calculate heat flux using the adjusted sensitivity.
7. Remove the sensor from the measurement surface if necessary while being very
careful not to damage it
Sensor Signal Measurement
The output of the HFS is a DC voltage that is linearly proportional to the heat flux absorbed
by the sensor. Similarly, the type-T thermocouple used for HFS surface temperature
measurements outputs a DC voltage proportional to the temperature difference between
the sensor surface and the voltage measurement location. The output DC voltage signals
can be measured with any precision voltmeter or 3rd party voltage data acquisition system
with a microvolt, μV, resolution. The construction design of the HFS allows for
measurement of both heat flux and temperature difference using four wires.
Measuring Heat Flux Voltage
To measure the sensor output voltage that is caused by the sensor absorbing heat
flux, ΔVq”, connect the positive (+) terminal of the voltage measurement device to the bright
red wire and negative (-) terminal to the white wire. The polarity of these wires does not
matter too much since heat flux will be positive or negative depending on the orientation
of the sensor.
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Measuring Thermocouple Voltage
A thermocouple is integrated in every HFS heat flux sensor to provide a
measurement of the sensor’s surface temperature. Measuring the voltage output from the
thermocouple will provide an indication of the temperature difference between the
temperature measurement location at the top sensor surface and the voltage
measurement location. An additional temperature measurement at the voltage
measurement location is required to determine absolute temperature of the sensor. This
reference temperature is known as cold junction compensation and is commonly built into
data acquisition systems.
Connect the darker red constantan wire to a negative (-) terminal of the voltage
measurement device. The positive (+) voltage measurement device lead should be
connected to the blue copper lead wire.
Checking the Functionality of HFS Sensor & Troubleshooting
There are a couple different simple tests can be performed to ensure that the HFS is
operating correctly. It is good practice to perform these before & after mounting the sensor
so that inaccurate measurements are not accidently taken using a faulty sensor that may
have been damaged while it was being handled.
1. Check Electrical Resistance of the Heat Flux Sensor:
Connect an ohmmeter to the bright red wire and the white wire leads to check the
heat flux sensor’s electrical resistance. It should <1000 Ω for HFS-5 sensors or <5
kΩ for HFS-6 & UHFS-09 sensors. If the resistance is much larger than this then
there may be an issue with the sensor. Resistance may be slightly larger for
sensors that have lead wire lengths longer than the standard 10 ft / 3 m. Infinite
electrical resistance (discontinuity) is an indication that the heat flux sensor is
broken.
2. Check the Electrical Resistance of the Thermocouple:
Connect an electrical resistance ohmmeter to the darker red constantan wire and
the blue copper wire. The electrical resistance should be approximately 50 Ω for
our standard 10 foot / 3 meter lead length. Resistance will be higher for longer lead
lengths. Typical resistance of the wire is 5 Ω per foot or 16 Ω per meter. Infinite
electrical resistance (discontinuity) is an indication that the thermocouple is
broken.
3. Check Output Voltage with Zero Heat Flux Through Sensor:
If possible, while a voltage measurement device is connected across the heat flux
wire leads, measure the output heat flux voltage while the sensor is experiencing
zero absorbed heat flux through the sensor. An easy scenario is to let the sensor
remain unmounted in ambient conditions such as sitting loose on a tabletop. The
analog DC voltage reading for the heat flux output voltage should be approximately
0.0 μV (+/- 5 μV can be contributed to electrical noise).
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4. Check the Output Voltage for an Induced Heat Flux Through
Sensor: A simple method of determining whether the HFS is roughly
operating correctly is by physically inducing a heat flux. Place the HFS
sensor on a metallic surface and firmly placing the palm of your hand across
the entire sensor surface. The resulting peak DC output voltage value
should be somewhere around 1.0 mV for a HFS (this value could vary over
20% from this value depending on the situation). You can also flip the
sensor over and a similar output DC voltage signal with an opposite sign
(positive (+) versus negative (-)) should be measured.
5. Check the Output Thermocouple Voltage:
If the sensor and voltage measurement location are at equal temperatures, the
output voltage from the thermocouple should be approximately zero microvolts
(this test scenario is often difficult to achieve easily). This test is rarely necessary
since thermocouple electrical resistance should be sufficient enough of a test.
6. Ensure the Sensor’s Serial Number Matches the Calibration
Certificate You will want to double check that the heat flux sensor’s serial
number matches that indicted on the Calibration Certificate. This will ensure
that you use the correct sensor sensitivity appropriate for your sensor. The
sensor’s serial number should be located on a tag on the wire leads.
Sensor Mounting/Installation
The manner in which the heat flux sensor is mounted depends on the application that it is
being used for. The best results are obtained from mounting the sensor on smooth clean
surfaces. The overall goal in mounting the HFS is to have the sensor strongly adhered
and fully contacting the measurement surface as uniformly as possible. This reduces the
amount of thermal contact resistance between the measurement surface and the sensor
and thus increases accuracy of measurements. Using one of the following mounting
techniques is recommended but can be adjusted according to the testing setup.
Mounting Method #1: Double-Stick Adhesive Tape
Commercially available double-stick tape is ideal for temporary mounting to solid
surfaces. When using double-stick tape, be sure the measurement surface is
clean, cover the desired mounting area with double sided tape, then firmly & evenly
press the sensor to the tape. If using multiple pieces of double stick tape, avoid
overlapping the tape on itself.
Mounting Method #2: Thermally Conductive Glue
Thermally conductive glue can be used for permanent mounting of the HFS. Before
mounting, clean the measurement surface and the surface of the sensor. Spread
a thin, uniform layer of the thermally conductive glue across the surface of the
sensor. Apply constant, uniform pressure to the sensor until the glue dries.
Removing the sensor from the surface after gluing it will likely destroy the sensor.
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Mounting Method #3: Thermally Conductive Paste
Thermally conductive paste is only appropriate if the sensor is being held in place
with a constant, uniform pressure while the sensor is taking measurements. An
example of an appropriate measurement scenario is if the sensor is used for
conductive heat transfer measurements while being placed between two surfaces
that are squishing it & holding it in place. Thermally conductive paste can be placed
between the sensor and each of the surfaces to minimize thermal contact
resistance. One recommended product is using OmegaTherm 201 conductive
paste available from Omega. Alternatively toothpaste has even been used when
nothing else is available and actually works fairly well.
Another method is to apply a thin layer of thermal conductive paste between the
sensor and the measurement surface. Then use an adhesive tape over the entire
sensor to keep it pressed down onto the surface.
Removing Sensor from Measurement Surfaces
Removing the HFS from the measurement surface is only recommended if a
temporary adhesive such as conductive paste or double-stick tape was used for mounting.
It is possible that higher strength adhesion methods can comprise the integrity of the
sensor if the sensor is removed from them.
IMPORTANT: When removing the sensor, very carefully remove the side with the leads
with one hand and peel the opposite side with the other hand to avoid bending as much
as possible. Slight bending of the sensor will not affect its performance but ripping it off
surfaces and forcing it to bend sharply should be avoided
Section 2: Converting Measurements to Heat Flux and Temperature
Type T thermocouple temperature measurement
Thermocouple temperature measurements can be recorded with a
thermocouple meter capable of reading T-type thermocouple, with cold junction
compensation. (Suggested meter: Omega DP41-TC)
Temperature Dependence of HFS Heat Flux Sensor
The output signals from HFS-5 and HFS-6 heat flux sensors have some
dependence on the temperature of the sensor itself. This dependence means that
sensor’s sensitivity changes slightly at different temperatures. UHFS-09 sensors do not
experience this dependence so ignore this section if using this sensor model.
Each sensor is calibrated at a base sensor temperature of 25°C or 77 °F. The
sensitivity at this temperature is recorded on the calibration sheet provided with each
individual heat flux sensor. An example of the calibration sensitivity, S
Calib
, is shown below
circled in red.
If you are using the heat flux sensor at a temperature that is different from 25°C or
77°F then it is suggested that you adjust the sensitivity to compensate for the temperature
dependence using the following steps.
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1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
M.F. = 0.00334*T
°
C
+ 0.917
Take the sensor’s temperature measurement, T
°C
, at each measurement in time
along with the calibrated sensitivity, S
Calib
, to determine the heat flux sensitivity at that
specific
tem
perature
, S
@
T
°C
.
𝑆
@ 𝑇
℃
= 𝑀. 𝐹.∗
𝑆
𝐶𝑎𝑙𝑖𝑏
= [0.
00334 ∗ 𝑇
℃
+ 0.917 ] ∗ 𝑆
𝐶𝑎𝑙𝑖𝑏
Sensitivity Multiplication Factor Versus
Sensor Temperature (25
°
C Origin)
-50 -30 -10 10 30 50 70 90 110 130
Sensor Temperature (°C)
Calculating Heat Flux
Using the DC voltage measurements taken from the heat flux wire leads (white &
bright red), the heat flux values can be calculated using the adjusted sensitivity for that
specific sensor and the following equation.
where q” is the heat flux absorbed through the sensor, ∆V
q”
is the HFS sensor output heat
flux voltage, and
S
@
T
°C
. is the s
ensitivi
ty of the
sensor that has been adju
sted
according
to the sensor’s temperature at that specific time.
For example: A voltage value of 1.80 mV is measured across the heat flux
leads, and the sensor’s calibrated sensitivity is specified to equal 0.90 µV/(W/m
2
),
and the sensor’s temperature is measured to be equal to 30°C from the
thermocouple leads at that point in time. The calculation for heat flux is as follows.
1. First, adjust the sensitivity according to the measured sensor temperature
from the sensor’s integrated Type-T thermocouple. (Ignore this step if using
the UHFS-09 sensor).
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2. Next, calculate heat flux using the adjusted sensitivity and voltage
measurement across the heat flux wire leads.
It should be noted that both positive and negative voltages can be measured by the HFS
sensor. A negative voltage value compared to a positive voltage just means that the heat
flux is moving in the opposite direction.
Heat Flux Sensor Sensitivity Determination
The sensor sensitivity is the output voltage induced by the sensor divided by the
heat flux conducted through the sensor.
Using a custom-made calibration apparatus, Heat flux can be calculated using the
measured temperature differential and a Standard Reference material’s known thermal
resistance.
The sensor’s sensitivity can be determined by dividing the output voltage from the sensor
by the heat flux.
The sensitivity can then be adjusted according to the temperature accordingly.
Where T
°C
is the sensor’s temperature in degrees Celsius & S
Calib
is the calibrated sensor
sensitivity provided in the table above.
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Section 4: Directive Compliances
RoHS3 Statement of Compliance Statement
For Directive (EU) 2015/863 of the European Parliament and of the
Council of 4 June 2015 on the restriction of use of certain hazardous
substances in electrical and electronic equipment.
The directive 2015/863 defines ten (10) substances which are to be restricted. The
maximum concentration by weight for each substance is listed below.
Substance
Maximum Concentration
1
Lead (Pb)
0.1%
2
Mercury (Hg)
0.1%
Cadmium (Cd)
0.01%
HexavalentChromium (Cr VI)
0.1%
Polybrominated biphenyls (PBB)
0.1%
Polybrominated diphenyl ethers (PBDE)
0.1%
Bis(2-ethylhexyl) phthalate (DEHP)
0.1%
Butyl benzyl phthalate (BBP)
0.1%
Dibutyl phthalate (DBP)
0.1%
Diisobutyl phthalate (DIBP)
0.1%
1
Restricted substances and maximum concentration values tolerated by weight in
homogeneous materials
2
Exemption 6(a) Lead as an alloying element in steel for machining purposes and in
galvanized steel containing up to 0.35 % lead by weight; Exemption 6(b) Lead as an
alloying element in aluminum containing up to 0.4 % lead by weight; Exemption 6(c)
Copper alloy containing up to 4 % lead by weight; and Exemption 7(c)-I Electrical and
electronic components containing lead in a glass or ceramic other than dielectric ceramic in
capacitors, e.g. piezoelectronic devices, or in a glass or ceramic matrix compound.
All HFS-5, HFS-6, & UHFS-09 heat flux sensor products will have the following
RoHS3 Compliance:
RoHS3 Status: Compliant
The RoHS Compliance of any product so designated is based upon evidence from the
producer (manufacturer) that the part number is in compliance with the RoHS Directive. All
reasonable steps have been taken to confirm producers' statements and other evidence
regarding the absence of the restricted substances to support the manufacturers' claim of
compliance. Based upon a review of the manufacturing records and technical information, this
product, to the best of our knowledge, does not contain any of the restricted substances in
quantities that exceed the limits, as specified above.
Approved By: Date:
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REACH Compliance Statement
For Directive EC 1907/2006 of the European Parliament and of the Council
of 18 December 2006 concerning the Registration, Evaluation,
Authorization and Restriction of Chemicals (REACH), establishing a
European Chemicals Agency, amending Directive 1999/45/EC and
repealing Council Regulation (EEC) No 793/93 and Commission Regulation
(EC) No 1488/94 as well as Council Directive 76/769/EEC and Commission
Directives 91/155/EEC, 93/67/EEC, 93/105/EC and 2000/21/EC.
The directive defines substances which are to be restricted. The REACH candidate list of
Substances of Very High Concern (SVHC) found at:
http://echa.europa.eu/chem_data/authorisation_process/candidate_list_table_en.asp
All HFS-5, HFS-6, & UHFS-09 heat flux sensor products will have the following
REACH Compliance.
REACH Status: Compliant
The REACH Compliance of any product so designated is based upon evidence from the
producer (manufacturer) that the part number is in compliance with the REACH Directive. All
reasonable steps have been taken to confirm producers' statements and other evidence
regarding the absence of the restricted substances to support the manufacturers' claim of
compliance. Based upon a review of the manufacturing records and technical information, this
product, to the best of our knowledge, does not contain any of the restricted substances in
quantities that exceed the limits, as specified above.
Approved By: Date:
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EU DECLARATION OF CONFORMITY
Company Name: Omega Engineering Inc.
Address: 800 Connecticut Ave, Suite 5N01, Norwalk, CT 06854
Telephone Number: 1-888-826-6342
Email Address: info@omega.com
We declare that the DoC is issued under the sole responsibility and belongs to the
following product:
Objects of the Declaration
Product Model Number: HFS-5 & HFS-6 & UHFS-09
The object of the declaration described above is in conformity with the relevant Union
harmonization legislation:
Directive 2014/32/EU
The following harmonized standards and technical specifications have been applied:
RoHS 2015/863
4 June 2015
EN50581:2012
1 November 2012
Signed for and on behalf of:
Omega
Engineering Inc.
2019-07-18
Rande Cherry CTO
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